Sorry you are having a tough time recently. I personally think it is best to write a list of things you want to say to the neurologist, that way you don't miss anything out.
Write down any changes in behaviour and give examples where possible.
Write down physical changes like walking problems, balance.
Hallucinations as well.
The more information your neurologist has, the better ,as he is prescribing the medication.
Being ill and admitted to hospital has taken your dad away from his normal routine and this can be unsettling for him.
With Parkinson's it is hard work to keep going and an additional illness can knock you for six. A person with a weakened immune system is going to take longer to recover than 'normally' expected to.
Hopefully your dad is still in recovery mode and will be back to his old self soon.
The neurologist can only help if he knows what the problems are.
Hope it all goes well for you, take care.